Needs for a carbon dioxide reduction, resource saving, and the like are promoting efficiency enhancement of power plants. For this purpose, active efforts are being made in gas turbine power plants to increase the temperature and pressure of a working fluid. In accordance with the increase of the temperature and pressure of the working fluid, to cope with an increase of stress by fluid force and a decrease of a material allowable stress due to the high temperature in rotor blades, stator blades, and so on, various attempts are being made also regarding their structures and so on.
In recent gas turbines, a cooling medium is supplied to hollow parts of rotor blades and stator blades with a hollow cooling structure fabricated by precision casting. This prevents the rotor blades and the stator blades from increasing in temperature due to heat transfer from working medium whose temperature and pressure are increased.
In the aforesaid precision casting, a ceramics core in the shape of the hollow cooling structure formed of ceramics is set in a mold, and metal is cast into the mold, and thereafter the residual ceramics core is dissolved and removed with a solution of a strong base, whereby a rotor blade or a stator blade having the hollow cooling structure is fabricated.
In the aforesaid gas turbine, the increase of the temperature and pressure of the working fluid increases stresses in vanes and sidewalls of the stator blades and so on. One method to reduce the stress in the blade is to enlarge an airfoil. Enlarging the airfoil, however, causes an increase of a flow loss of the blade and is not desirable in view of thermal efficiency of the gas turbine.
In a typical gas turbine, the stress is reduced by a grouped blade structure in which a plurality of airfoils are set in one structure. This grouped blade structure, however, is typically employed in an uncooled stage and a rear stage having a simple cooling structure. Reasons for this are a heat stress and manufacturing limit of the precision casting, and a stator blade having a complicated cooling structure has no choice but to have a segment structure in which one airfoil is set in one structure. However, a structure that is not the grouped blade structure is weak against torsional deformation force of the blade, and its airfoil has to be enlarged at the risk of a decrease of thermal efficiency of the gas turbine.
As one method to decrease a stress in a steam turbine, there is known a method to weld an inner ring to a shroud to integrate them at the time of assembling a stator blade. However, a gas turbine stator blade used in a temperature range necessitating the cooling of the stator blade comes to have a temperature variation among their components and parts when it is cooled, and suffers a heat stress and thus is not suitable for the integrated structure.